This invention relates to the field of radar signal processing for a doppler batch synthetic aperture radar (SAR). and especially to the compensations used to provide return signals of equal amplitude from diversely located target points in the presence of antenna beam pattern and range attenuation effects.
In previous SAR ground mapping systems corrections for beam shape and range effect have been accomplished in a variety of ways, the most common of which involves selective spoiling of the radar beam. In this common prior arrangement, a cosecant squared beamshape is often employed in order to accommodate range and beam-shape attenuation affects in the radar return signal. Beam spoiling is, however, a highly complex scheme when employed in a phased array antenna radar system and excessive beam spoiling results in large doppler ambiguity effects and significant losses of system gain. Nevertheless, beam spoiling is often employed, especially in the elevation direction of a SAR ground mapping system, and usually in the form of representative beamshape correction curves for a variety of mapping geometry situations. Usually these corrections involve the return signal being multiplied by a range correction factor. Imperfections in these prior gain corrections, however, give rise to the need for correction arrangements suitable for use with complex antennas and beam patterns which include significant gain falloff toward the edge of a mapped region.
It is desirable for an improved gain correction arrangement to be adaptable to a variety of beamshaped geometries that are within the performance window of a doppler SAR system, and for the gain corrections to be generated in a small time period--a period having minimal impact on the frame time of the radar system. It is also desirable for the gain correction to be applicable to more than one resolution cell of a mapped area in order to minimize correction memory requirements and for the gain correction factors to be smooth in nature so there are no large discontinuities in the mapped area and no large roll off from one side of the mapped area to another.